During the rolling of metal strips, and particularly of cold rolled thin strip, planarity defects appear, due particularly to different relative reduction transversely thickness of the strip to the rolling direction. These parts which undergo different reductions have a tendency to elongate likewise in different ways, and a state of internal stress therefore occurs within the metal sheet which, when the material is not sufficiently thick and rigid, is expressed in planeity defects such as pockets in the center of the metal sheet or waves on its edges.
These planeity defects must be detected in order to remedy them by influencing the rolling conditions, and in any case in order to monitor the quality of the product at the end of the operation. For this purpose, a deflecter roller is generally used, over which the metal sheet to be monitored is passed under tension, this monitoring roller generally consisting of a tubular wall mounted rotatably about an axis perpendicular to the direction of travel of the strip, and on the circumference of which an array of pick-ups are placed, arranged in at least one series comprising a plurality of pick-ups distributed over the length of the roller in a direction substantially parallel to the axis of rotation.
The pick-ups are fitted detachably in housings made in the tubular wall, and normally open on the interior face of the latter, the end of the housing facing the exterior being enclosed by a thin envelope which may be applied on the tubular wall by hooping. In order to facilitate the fitting of the pick-ups, the latter may be threaded into orifices machined in the interior wall and each fixed on the latter by a flange.
In a further embodiment, the pick-ups are arranged in a plurality of mutually spaced series so as to cover the circumference of the roller. In each series, the pick-ups may advantageously be spaced slightly on either side of a mean direction parallel to the axis of the roller, and their longitudinal positions are moreover determined so that they are distributed along a helicoidal line winding in a plurality of turns over the entire length of the roller thus, the pick-ups cover the entire width of the metal sheet, passing successively beneath the part of the latter resting upon the roller during one or more turns of the latter.
In order to detect accurately the planarity defects resulting from the state of stress detected by the pick-ups, the latter must be positioned and oriented within their respective housings with great precision, adjusting the zero of each one. Furthermore, considering the pick-ups placed along one and the same generatrix of the roller, it is preferable to effect a slight angular offset of the pick-ups relative to each other, and since the pick-ups are normally fitted through the interior of the interior tubular wall of the roller, their precise fitting and adjustment constitutes a long and onerous operation. Moreover, generally speaking, the maintenance of the whole assembly is rather difficult and expensive.